The manufacturing of a composite structure may include applying uncured resin to reinforcing fibers of a composite layup. The temperature of the composite layup may be increased to reduce the viscosity of the resin so that the resin may flow and infuse the fibers. The composite layup may be held at an elevated temperature for a predetermined period of time to cure the resin into a solidified or hardened state. After the resin has cured, the composite structure may be passively or actively cooled to ambient temperature.
In many composite material systems, the resin may have a coefficient of thermal expansion (CTE) that may be different than the CTE of the reinforcing fibers. For example, epoxy resins may have a CTE that may be an order of magnitude greater than the CTE of carbon fibers. The difference in CTE may result in the resin and fibers contracting by different amounts as the composite structure cools down from the curing temperature. The difference in contraction of the resin relative to the fibers may result in thermally-induced stresses in the resin. The thermally-induced stresses may result in undesirable cracking or microcracking in the resin. Microcracking may also occur during the service life of a composite structure due to changes in temperature of the operating environment of the composite structure.
Prior art attempts to reduce or prevent microcracking include the addition of tougheners to liquid resin. Conventional thermoset resins may be formed using liquid polymers to form an uncured liquid resin. Alternatively, solid polymers may be dissolved into liquids during mixing to form an uncured liquid resin. Tougheners in liquid form may be added to the uncured liquid resin to improve the resistance of the resin to microcracking Unfortunately, adding liquid tougheners to resin may result in a reduction in the final resin glass transition temperature during curing, or the liquid tougheners may increase the cure temperature of the resin and/or cause excessive cure shrinkage of the resin. In addition, tougheners often increase the viscosity of the resin which may impair manufacturability and thus effectively limit the amount of toughener that can be added to the resin. Advanced thermoset resins typically require relatively high cure temperatures (e.g., 350-600° F.) to fully cure the thermoset resin/composite. Such high cure temperatures may result in increased thermally-induced stresses and strains due to the differential CTE between the fibers and the resin.
As can be seen, there exists a need in the art for a system and method for improving the properties of a resin such as resin toughness, and which avoids one or more undesirable characteristics including, but not limited to, high heat of reaction, high cure temperatures, and excessive cure shrinkage of the resin.